Secondary organic aerosol

A secondary organic aerosol (SOA) is a molecule produced via oxidation over several generations of a parent organic molecule.[1] In contrast to primary organic aerosols, which are emitted directly from the biosphere, secondary organic aerosols are either formed via homogeneous nucleation through the successive oxidation of gas-phase organic compounds, or through condensation on pre-existing particles. These gas-phase species exert high vapor pressures, meaning they are volatile and stable in the gas-phase. Upon oxidation, the increased polarity, and thus reduced volatility, of the molecules results in a reduction of vapor pressure. After sufficient oxidation, the vapor pressure is sufficiently low that the gas-phase compound partitions into the solid-phase, producing secondary organic matter (the particle phase of secondary organic aerosol).

SOAs represent a significant proportion of aerosols contained in the troposphere.[1]

A common misconception is that the aerosol refers to the solid phase of the compound, where in reality, by definition, it is the combination of the gas- and solid-phases which constitute the aerosol.